Novel Mutations in B3GALNT2 Gene Causing Alpha-Dystroglycanopathy in Chinese Patients
Alpha-dystroglycanopathy (α-DGP) is a subtype of congenital muscular dystrophies (CMDs) characterized by autosomal recessive inheritance. The primary pathogenesis of α-DGP involves defects in the post-translational O-glycosylation of alpha-dystroglycan (α-DG). This condition presents a wide clinical spectrum, ranging from severe forms such as Walker-Warburg syndrome (WWS), muscle-eye-brain disease (MEB), and Fukuyama congenital muscular dystrophy to milder forms like limb-girdle muscular dystrophy. The B3GALNT2 gene, which encodes an enzyme critical for the production of a unique carbohydrate structure (GalNAc-β-1,3-GlcNAc), is one of the pathogenic genes associated with α-DGP. This carbohydrate structure is essential for the proper O-glycosylation of α-DG. Mutations in B3GALNT2 have been linked to severe clinical manifestations, including WWS and MEB, which are often associated with a short lifespan, severe muscle weakness, ocular anomalies, and brain malformations.
This study describes two Chinese patients with compound heterozygous mutations in the B3GALNT2 gene, expanding the mutational and phenotypic spectrum of α-DGP. The research protocol was approved by the Ethics Committee of Peking University First Hospital, and written informed consent was obtained from the patients and their parents.
Patient 1
Patient 1 was a 2-year and 11-month-old girl, the second child of healthy, non-consanguineous parents with no family history of neuromuscular disorders. She was born at full-term without perinatal complications. At 10 months of age, she exhibited poor interaction with others and was unable to stand independently until the age of 2. Physical examination at 13 months revealed a head circumference of 44.5 cm (below the 25th percentile), proximal muscle weakness, and decreased tendon reflexes without joint contractures. By the time of the study, she could walk independently for a few minutes but had a wide-based gait and was unable to understand instructions or speak complex words. A neuropsychological evaluation at 10 months of age indicated a developmental quotient (DQ) of 66, signifying mild to moderate mental retardation. Serum creatine kinase (CK) levels, auditory tests, and fundus examinations were normal. Brain magnetic resonance imaging (MRI) at 11 months of age showed polymicrogyria, increased T2 signal intensity in the white matter of the cerebrum and cerebellum, mildly enlarged ventricles, cerebellum cysts, and dysplastic cerebellum and brainstem.
Patient 2
Patient 2 was a 1-year and 9-month-old girl, the first child of healthy, non-consanguineous parents. She was born at full-term without perinatal complications. She began speaking one- or two-syllable words at 8 months, stood with assistance at 1 year, and walked with support at 15 months, but showed no further progress thereafter. She had no ocular or auditory impairments. A psychological evaluation at 15 months of age revealed a DQ of 57, indicating moderate to severe mental retardation. Physical examination at 17 months showed a head circumference of 45 cm (below the 25th percentile), decreased muscle strength, and no joint contractures. Knee reflexes were present bilaterally. Serum CK levels at 17 months were elevated at 565 U/L (normal value <170 U/L). Brain MRI at 7 months of age demonstrated mildly increased T2 signals in the white matter of the cerebrum, brainstem, and cerebellum dysplasia, along with mildly enlarged ventricles.
Genetic Analysis
Whole exome sequencing was performed on both patients, and candidate variants were validated by Sanger sequencing in their families. Copy number variants (CNVs) were confirmed by quantitative polymerase chain reaction if next-generation sequencing data revealed differences in sequence reads between patients and control samples. Both patients were found to carry compound heterozygous mutations in the B3GALNT2 gene, with their parents being carriers.
Patient 1 had compound heterozygous variants of c.979G>A (p.D327N) and c.1421_c.1423delCTC (p.P474del), which had been previously reported. No CNVs were detected in this patient. Patient 2 harbored novel compound heterozygous variants of c.48dupG (p.L17fs) and c.1183G>A (p.G395R). Additionally, she had a CNV involving a duplication of 550 kilobase pairs on chromosome 12 (94075229-94625165). According to the American College of Medical Genetics and Genomics and the Association for Molecular Pathology guidelines, the novel variants were predicted to be pathogenic and likely pathogenic, respectively. The CNV was not recorded in the chromosome CNVs polymorphism database of the normal population, and no smaller pathogenic duplications in this region were found in the Database of Genomic Variants and Phenotype in Humans using Ensembl Resources (DECIPHER). Therefore, the pathogenicity of the CNV was considered uncertain.
Clinical Phenotypes and Genotype-Phenotype Correlation
Both patients exhibited early-onset muscle weakness, psychomotor and language retardation, microcephaly, and brain structural abnormalities without ocular anomalies. Their clinical presentations were milder than typical MEB, leading to the classification of their condition as MEB-like phenotypes.
The B3GALNT2 gene, located on chromosome 1q42.3, consists of 12 exons and encodes approximately 500 amino acids. The protein includes one transmembrane domain, one stem region, and a galactosyltransferase catalytic domain, which spans amino acids 307 to 457. The severity of clinical manifestations in α-DGP patients with B3GALNT2 mutations appears to depend on the degree of impairment in protein function rather than the specific region of the mutation.
For instance, five Iranian siblings with homozygous p.D327N mutations exhibited milder symptoms than Patient 1, with psychomotor and language retardation, non-specific cerebral white matter changes, and epilepsy but no brain structural deformities or ocular anomalies. In contrast, a Swedish patient with p.D327N and p.E65fs mutations had a phenotype similar to Patient 1. The p.P474del mutation, combined with p.S25Cfs38, has been associated with severe MEB. Although p.P474del is located outside the catalytic domain, it is a conserved site across species, suggesting that it may contribute significantly to the severity of clinical manifestations in Patient 1.
Patient 2’s novel mutations, p.G395R and p.L17fs, are located in the catalytic and transmembrane domains, respectively. Both sites are conserved, and the frameshift mutation likely results in a truncated protein. These findings highlight the complexity of genotype-phenotype correlations in α-DGP and the need for further studies to elucidate the molecular mechanisms underlying these mutations.
Conclusion
This study reports two Chinese patients with novel compound heterozygous mutations in the B3GALNT2 gene, expanding the mutational and phenotypic spectrum of α-DGP. The clinical presentations of these patients, characterized by MEB-like phenotypes, underscore the variability in the severity of α-DGP associated with B3GALNT2 mutations. The relationship between specific mutations and their effects on enzyme activity remains unclear, necessitating further research to understand the pathogenicity of these mutations and their molecular mechanisms.
doi.org/10.1097/CM9.0000000000001283
Was this helpful?
0 / 0